Communication system



April 27, 1943. D. c. M RAE COMMUNICATION SYSTEM Filed Sept, 6, 1941 m 3 6 H 7 0 6 pp, 8 2 1 f /1 MW rm 5 m 1.1M 5km L W 7 l 7 MW 5 w i A rm A E c. M 2 9 M A 4 6 4 A m M mm A MW 0V T M Z 2 y H ATTORNEY.

Patented Apr. 27, 1943 UNITED STATES COMW IUNICATION SYSTEM Don C. McRae, Miami Springs, Fla.

Application September 6, 1941, Serial No. 409,816

1 Claim.

My invention relates to radio communication systems, and more particularly to an improved system for two-way radio communication.

Two-way communication between aircraft and ground stations is now in wide use. In its present form, a superheterodyne receiver and a transmitter are located in the aircraft and, during operation, the receiver is tuned to the receiving channel, While the transmitter is tuned to the transmitting channel. In domestic commercial airline service, i. e., service within the confines of this country, the frequency of the receiving channel and the transmitting channel are normally identical and communication in both directions takes place on a single channel. The channel frequencies are predetermined and the equipment is maintained on the proper frequency by means of quartz crystal frequency determining elements, one crystal for the receiver oscillator and one for the transmitter oscillator. Thus, two crystals are required for each channel. A disadvantage of this system is that in multi-channel equipment these crystals may readily come to occupy an appreciable part of the total bulk and greatly increase the cost of the complete system.

In transoceanic airline service, where very great radio transmission distances are involved, the frequency of the transmitting channel and th frequency of the receiving channel are not, in general, the same, because the optimum frequency for 3 transmission in one direction may not be the same as the optimum frequency for transmission in the other direction. These frequencies are usually predetermined and preset, one crystal being employed for each receiving channel and one crystal being employed for each transmitting channel. In multi-channel systems, this arrangement also has the disadvantage of requiring the use of a large number of crystals.

Military aircraft require a still more flexible I craft to aircraft and ground station to ground station communication present lik requirements and like disadvantages.

One of the principal objects of this invention is to provide a communication system in which'the number of precise frequency determining elements required is reduced from previous practice.

Another object of this invention is to provide a two-way communication system in which adjustment of the receiver to the receiving channel frequency automatically controls the transmitting channel frequency.

Still another object of this invention is to provide two-way communication apparatus in which the number of tuning controls to be manipulated by the operator, is reduced.

Other objects and advantages will in part be disclosed and in part be obvious when the following specification is read in conjunction with the drawing, in which:

Figure 1 is a block diagram showing one em bodiment of the invention; and Figure 2 is a block diagram showing an alternative form of the invention. 1

Referringin more detail to Figure 1'; a receiver l and a transmitter l5 may be situated in either a fixed or mobile location. Receiver l is of the superheterodyne type having a radio frequency amplifier 2 connected to the mixer stage 3. Also connected to mixer stage 3 is an oscillator '4. The mixer stage output resulting from the combined action of the signal and oscillator voltages is impressed on the input circuit of the intermediate frequency amplifier 5 and the output of this amplifier 5 may, in turn, be fed effectively into succeeding detector and amplifier stages (not shown) designed to utilize this energy. Situated in transmitter I5, is a second mixer stage 1 which receives energy from oscillator via the lead 6. and, in addition, receives energy from a'second oscillator 8, also located in transmitter 15. The output of mixer stage I, resulting from the combined action of oscillator 4 and oscillator 8', is fed to the radio frequency amplifier 9, where the desired frequency component is selected and amplified, and then conveyed to a modulator I0. A send-receive switch H alternatively connects the antenna [2 to the input of radio frequency amplifier 2, in the receive position, or to the output of modulator Iii when in the send position, at the same time grounding the antenna con nection of the apparatus not in use. An additional section may be added to switch I l and arranged to interrupt the anode supply for radio frequency amplifier 9 and modulator l [I when the switch I l is in the receive position, thereby protecting the vacuum tubes in the transmitter portion of the apparatus from damage due to excessive anode current.

Operation of the equipment takes plac in the manner described: With switch I I in receive position, the signal frequency f1 is fed from antenna I2 to the input of radio frequency amplifier 2, and thence to the input of mixer 3. The output frequency 1: of oscillator 4 is also connected to mixer 3. These two frequencies are combined through the action of mixer 3, so that there appears in the mixer output, in addition to the frequencies f1 and f2, the frequencies f2+fi and f2-f1. These frequencies are fed to the input of intermediate frequency amplifier 5. The most frequent practice in the contemporary art is to tune the intermediate frequency amplifier 5 to respond to the lower of these two latter fre quencies, in which case there appears in the output of this amplifier the frequency f2f1=f3. All the above parts are already known in combination, and together they constitute the input and frequency changing system of a well known type of receiver. In practice, intermediate frequency amplifier '5 is permanently tuned to is, which is some fixed predetermined frequency, and the frequency of oscillator 4 is varied until the maximum transmission gain is obtained through intcrmediate frequency amplifier 5, in which case f2 f1=fs. With switch II in the "receive position, the output of modulator I0 is short circuited to ground, and consequently the operation of receiver I is unaffected by transmitter I5.

With switch I I in the send position, the input of radio frequency amplifier 2 is short circuited to ground, thus rendering the receiver inoperative. It will be remembered, however, that the receiver has been tuned for maximum response to the signal frequency f1, and therefore oscillator 4, which continues operation, is oscillating at a frequency ,fz, such that f2- f1=f3. The output frequency f2 of'oscillator 4 is fed to mixer I. Also fed to mixer I is the output frequency of oscillator 8, which is fixedly tuned to the frequency of maximum response of intermediate frequency amplifier 5, i. e. f3=f2-f1. As a result of the action of mixer I, there appears in its output not only the impressed frequencies, but also sum and difference frequencies, namely f1 and 2fzf1. Radio frequency amplifier 9 is tuned to respond to frequency if, so that there appears in its output the frequency h, which is fed to the radio frequency input circuits of modulator ID. The F output of modulator Ill, modified in the desired manner, is then fed to antenna I2 and radiated into space.

In the practice of this invention, oscillator 8 is preferably of the fixed frequency, highly stable type. The frequency of oscillator 4 may be fixed, and maintained by a quartz crystal element, or the equivalent thereof, if the equipment is intended for use on several permanently assigned channel frequencies. Should the need exist for ready alteration of th channel frequencies to any value, oscillator 4 may be of a type having its frequency determined by an LC circuit in which L and/or C are varied as desired. In this event, maximum accuracy of the transmitted frequency is insured by adding the connection I3, between the oscillator 8 and the intermediate frequency amplifier 5. In making use of this arrangement, the receiver is first tuned to the desired signal and the frequency of oscillator 4 is then adjusted until a zero beat is obtained in the receiver as a result of the beating of the intermediate frequency with oscillatory energy from oscillator 8. As this connection is preferably utilized only during the initial tuning operation, the switch I6 is provided in the line I3 to interrupt nergy transfer along this line after completing the adjustment of oscillator 4.

By designing oscillator 8 so that its frequency is not the same as the intermediate frequency of the receiver, the transmitted frequency may be made different than the received frequency, although it will still be variable upon tuning of the oscillator 4.

Referring to Figure 2, a modified form of the invention is shown, in which the transmitted frequency is a multiple of the receiving frequency. This form of the invention differs from that shown in Figure 1 only in the insertion of multiplier I 4 between mixer I and radio frequency amplifier 9 indicated in the Figure 2. This multiplier multiplies the frequency of a selected beat in the output of the mixer I by the factor n. All the modifications of the invention discussed under Figure 1 may be applied with equal readiness to the form of Figure 2.

It will be obvious that many changes and modifications may be made in the invention without departing from the spirit thereof as expressed in the foregoing description and in the appended claim.

I claim:

In a carrier communication system, a mixer stage, means for impressing electrical energy at a predetermined signal frequency on said mixer stage, an oscillator having an output frequency continuously variable over a predetermined range of frequencies, means for impressing energy derived from said oscillator on said mixer stage, means for amplifying and detecting one of the resultant beat frequency energies present in the output of said mixer stage, a second oscillator providing energy at a frequency substantially equal to that of the amplified and detected beat frequency energy, means for impressing energy from said second mentioned oscillator on said amplifying and detecting means, a second mixer stage, means for impressing energy from first mentioned oscillator on said mixer stage, means for impressing energy from said second mentioned oscillator on said second mixer stage, means for selectively amplifying one of the resultant beat frequency energies present in the output of said second mixer stage, and means for transmitting said amplified energy.

DON C. McRAE.

CERTIFICATE OF CORRECTION.

Patent No. 2,517,5li7. April 27, 19L 5.

' DON c. MCRAE.

It is hereb; certified that error appears in the above numbered patent requiring correction as follows: In the drawing, Figures land 2 should appear as shown below instead of as shown in the patent F/G. I

RECIEIVE SEND f-f 'f 5 f; 7% 7? 5-5, 2 Z M/Xfk osc.

i MODULATING- VOLTAGE Q M ODULAT/NG VOLTA GE and that the said Letters Patent shouldbe read with this correction therein that the same may conform to the record of the case in the Patent Office;

Signed and sealed this 5th day of October, A. D. 1915.

Henry Van Arsdale, (Seal) Acting Commissioner of Patents. 

